In the related art, PTL 1 discloses a corrugated sheet folding device as a sheet folding device that folds a corrugated sheet. The folding device includes a folding belt and a folding bar. The folding belt folds both end portions of the corrugated sheet in a width direction. The folding bar assists in the folding operation by the folding belt. The folding device can fold a plurality of types of the corrugated sheets that differ in shape, size, and the like, and an additional folding bar is mounted when corrugated sheets of special specifications are folded. In this manner, the folding device can also fold the corrugated sheets of special specifications appropriately.